Metastasis is a complex process that consists of a series of steps including invasion, intravasation, extravasation and growth to form micrometastasis in other tissues. Thus far, not much is known about what triggers cancer cells to invade and intravasate. Knowing more about these early steps of metastasis would help identify therapeutic targets to aid in the prevention of metastasis. To increase our current knowledge on the processes of invasion and intravasation the following specific aims are proposed: Aim I. Investigation of the in vivo invasive behavior of MTLn3-ErbB3 cells. ErbB3 overexpression has been shown to increase the metastatic behavior of MTLn3 cells in vivo as well as their chemotactic response to HRG[unreadable]1 in vitro;therefore this cell line provides an opportunity to evaluate, using the in vivo invasion assay, whether there are other signaling pathways that regulate in vivo invasion aside from the published EGF/CSF-1 paracrine loop, as well as to assess the importance of macrophages for cancer cell invasion. Aim II. Determination of the "intravasation signature." Because not all cancer cells intravasate, those cancer cells that undergo the process must undergo gene expression changes indicative of the process. To begin to elucidate the gene changes occurring in breast cancer cells that intravasate, the global gene expression of intravasated MTLn3 cells will be compared to that of cancer cells remaining in the primary tumor using microarray analysis. The resulting "intravasation signature" will be compared to the existing invasion signature to assess the similarities/differences between these processes. Aim III. Characterization of the function of key genes in intravasation. To test the biological validity of the "intravasation signature", key genes from the "intravasation signature" will be selected to test their role in metastasis. Because motility has been previously characterized as an important feature of metastatic cancer cells, the effect of the selected genes in MTLn3 cells'ability to extend lamellipodia and chemotax will be evaluated. To test the genes'effect in MTLn3 metastatic behavior in vivo, blood burden and lung metastases will be determined. Metastasis is the number one cause of mortality in cancer patients. This project will increase our understanding of what triggers cancer cells to start the complicated process of metastasis, and will result in the generation of novel targets for therapies to aid in the prevention of metastasis